The present invention relates to an injection molding machine in which a servo motor for driving an injection mechanism controls a back pressure applied to a molten resin during metering and a back pressure control method of the same. More particularly, the present invention is an injection molding machine which can correctly control the value of the back pressure for a wide range and a back pressure control method of the same.
In the metering process of an injection molding machine, a resin as a molding material is plasticized to a molten state in a heating cylinder by rotation of a screw, and the molten resin is then stocked in the distal end portion of the heating cylinder. The screw is moved backward by the pressure of the molten resin. When the screw is moved backward to a preset metering position, the rotation of the screw is stopped, and the metering process is ended.
Conventionally, in order to adjust the melting and metering effect for the resin, the back pressure is controlled during metering. For example, in an injection molding machine of a type in which an injection mechanism is driven by hydraulic pressure, the back pressure is also controlled by the hydraulic pressure. Recently, in a molding machine of a type in which an injection mechanism is driven by a servo motor, a method has been developed to perform back pressure control by applying a torque limit to the servo motor (refer to Japanese Patent Application No. 60-88911).
In an injection molding machine using a servo motor, the injection servo motor is driven so that the screw is kept at the present position during metering. However, as the screw rotates, the resin pressure is increased to move the screw backward, thereby causing a positional error. In this case, a drive command is output to the servo motor to return the screw to the initial position. However, since the drive current of the servo motor is limited by a torque limit value, corresponding to the preset back pressure, a force exceeding the preset back pressure is not applied to the resin.
In order to apparently eliminate the positional error caused as a result of this, a numerical controller detects the value of an error register, and subtracts the detected value from the error register, thereby setting the register value to zero, in other words, following up the value of the error register. In practice, however, a delay occurs in the time required for the numerical controller to complete follow-up of the value of the error register. Therefore, the screw is moved backward during this delay time. Even if the value of the error register is followed up, the error register is not actually set to zero. As a result, a motor drive current command corresponding to the torque limit value is always output, and the motor output torque is in accordance with the torque limit value. When the screw is moved backward to reach the preset metering position, the rotation of the screw is stopped, thereby completing the metering process.
However, with a conventional servo-type back pressure control system, back pressure control cannot be correctly performed because of the reasons to be described later.